The present invention relates to a system for controlling the shift of an automatic transmission and, more particularly, to a system for controlling an oil pressure for executing the shift.
As is well known in the art, an automatic transmission for vehicles is constructed to change power transmission paths in a gear train to execute a shift by applying/releasing frictional engagement means including clutches and brakes. Since the shifting operation is followed by rotational fluctuations of the rotary elements including the engine, the output shaft torque is abruptly changed to cause a shifting shock thereby to deteriorate the riding comfort, if the frictional engagement means is abruptly applied or released. In the case of the so-called "clutch-to-clutch shift", in which a predetermined frictional engagement means is released whereas another is applied, the engine will be either unloaded or overloaded to lower the output shaft torque and the durability of the frictional engagement means if the applying or releasing timing is improper.
In the prior art, the apply pressure of the frictional engagement means for absorbing the inertial energy accompanying the rotational fluctuations of the rotary elements is gradually augmented according to the characteristics of an accumulator by attaching this accumulator to the frictional engagement means.
If the oil pressure is fed to the frictional engagement means through the accumulator, it is possible to maintain the supply pressure at a low level till the packing clearance is filled up or to raise the apply pressure gently after the frictional engagement means has taken the torque capacity. Since, however, the accumulator having such actions has to be given a volume sufficient for containing considerably much oil, it is desired for reducing the size and weight of the hydraulic control system to control the apply pressure of the frictional engagement means directly by a pressure regulating valve in place of the accumulator.
In case the oil pressure is to be fed to a predetermined one of the frictional engagement means so as to execute the shift, it is necessary to perform continuous controls of a maintenance of the low-pressure state till the packing clearance is filled up, a relatively abrupt rise of the oil pressure till the start of an inertial phase, a gentle rise of the apply pressure after that, and a pressure rise after the end of the shift. In case these controls of the oil pressure are to be carried out by a mechanism including the pressure regulating valve, there are used a plurality of devices including not only the pressure regulating valve but also a solenoid valve for controlling the regulated pressure level or an electronic control unit. As a result, not only the degree of freedom in the pressure regulation but also the occurrence of failures become far more probable than those of the case in which the pressure is regulated only by the accumulator. If some device is failed, a high apply pressure is abruptly fed to the frictional engagement means, and the shifting shock may possibly deteriorate.
In the prior art, moreover, a one-way clutch is employed for smoothing the shift. At a gear stage to be set by applying the one-way clutch, the engine braking cannot become effective so that a multi-disc clutch or a multi-disc brake has to be arranged in parallel with the one-way clutch. If, therefore, the one-way clutch could be eliminated, the automatic transmission can have its size and weight reduced, but no control is required for effecting the engine braking. If, moreover, the one-way clutch is eliminated, the timings of applying/releasing the aforementioned two frictional engagement means have to be controlled by controlling the individual oil pressures to be fed to and released from the frictional engagement means. For this necessity, there is provided a hydraulic system which can control the oil pressures of those frictional engagement means independently of each other. Thus, the degree of freedom of the oil pressure control can be enhanced to provide a shift control suited better for the running state of the vehicle.
If, however, the two frictional engagement means participating in the shift could have their oil pressures controlled independently of each other, a more precise control could be obtained, as described above. Because of the independent control systems, on the contrary, the two frictional engagement means might be simultaneously fed with the oil pressure due to some failure or a dispersion of the quality of the control device. If such tie-up of the two frictional engagement means should take place, continue for a long time or occur frequently, the frictional engagement means might have durability degraded due to their individual excessive slips.